Aqueous developable dual switching photoresists for nanolithography

Article abstract of DOI:10.1002/pola.26232

Photon-mediated switching of polymer solubility plays a crucial role in the manufacture of integrated circuits by photolithography. Conventional photoresists typically rely on a single switching mechanism based on either a change in polarity or, molecular weight of the polymer. Here we report a photoresist platform that uses both mechanisms. The molecular weight switch was achieved by using a poly(olefin sulfone) designed to undergo photo-induced chain scission. The polarity switch was achieved using pendant groups functionalized with o-nitrobenzyl esters. These are hydrophobic photosensitive-protecting groups for hydrophilic carboxylic acids. On irradiation, they are cleaved, making the polymer soluble in aqueous base. Importantly, the resists do not contain photoacid generator, so do not suffer from problems associated with acid diffusion that are detrimental to pattern fidelity. The 193 nm photochemistry of polymer thin films was followed using grazing angle attenuated total reflectance Fourier transform infrared spectroscopy, variable angle spectroscopic ellipsometry, and measurements of solubility in aqueous base. The nanoscale patterning performance of the polymers was also assessed using 193 nm interference lithography and electron-beam lithography. The implications of using dual switching mechanisms are discussed.

Full text of DOI:10.1002/pola.26232

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Aqueous Developable Dual Switching Photoresists for Nanolithography
Lan Chen,1 Yong Keng Goh,1 Han Hao Cheng,2 Bruce W. Smith,3 Peng Xie,3
Warren Montgomery,4 Andrew K. Whittaker,1,5 Idriss Blakey1,5
1The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, St Lucia, Queensland 4072, Australia
2The University of Queensland, Centre for Microscopy and Microanalysis, St Lucia, Queensland 4072, Australia
3Rochester Institute of Technology, Microsystems Engineering, 77 Lomb Memorial Drive, Rochester, New York 14623
4Prior CNSE Assignee to SEMATECH, Lithography Division, Albany, New York (returned to the College of Nanoscale Science and
Engineering (CNSE), 255 Fuller Rd. Albany, New York 12203)
5The University of Queensland, Centre for Advanced Imaging, Queensland 4072, Australia
Correspondence to: I. Blakey (E-mail: i.blakey@uq.edu.au)
Received 4 April 2012; accepted 21 June 2012; published online
DOI: 10.1002/pola.26232
ABSTRACT: Photon-mediated switching of polymer solubility
plays a crucial role in the manufacture of integrated circuits
by photolithography. Conventional photoresists typically rely
on a single switching mechanism based on either a change
in polarity or, molecular weight of the polymer. Here we
report a photoresist platform that uses both mechanisms.
The molecular weight switch was achieved by using a poly(o-
lefin sulfone) designed to undergo photo-induced chain scis-
sion. The polarity switch was achieved using pendant groups
functionalized with o-nitrobenzyl esters. These are hydropho-
bic photosensitive-protecting groups for hydrophilic carbox-
ylic acids. On irradiation, they are cleaved, making the
polymer soluble in aqueous base. Importantly, the resists do
not contain photoacid generator, so do not suffer from prob-
lems associated with acid diffusion that are detrimental to
pattern fidelity. The 193 nm photochemistry of polymer thin
films was followed using grazing angle attenuated total re-
flectance Fourier transform infrared spectroscopy, variable
angle spectroscopic ellipsometry, and measurements of solu-
bility in aqueous base. The nanoscale patterning performance
of the polymers was also assessed using 193 nm interference
lithography and electron-beam lithography. The implications
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of using dual switching mechanisms are discussed.
2012
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 000:
000–000, 2012
KEYWORDS: copolymerization; imaging; photochemistry; photo-
reactive effects; photoresists; structure–property relations
INTRODUCTION Photolithography is a critical step for manu-
facturing integrated circuits (ICs),1 including digital devices
such as microprocessors, memory, and systems on a chip.
Analogue devices such as sensors, power management cir-
cuits, and operational amplifiers are also manufactured using
this technology. Finally, photolithography is also used in the
fabrication of other devices including photomasks,2 plas-
monic devices,3 micro/nano-fluidic devices,4 micro/nano-
electromechanical systems (M/NEMS),5,6 gene chips,7 and
protein microarrays.8 Briefly, the process relies on having a
suitably designed polymer that is blended with a small
amount of a photoacid generator (PAG), which when exposed
to light can undergo photoreaction to form a strong photoa-
cid.9 This photoacid catalyzes the deprotection of pendant
hydrophobic tertiary esters or carbonates in the polymer,
yielding hydrophilic carboxylic acids or phenols. The depro-
tection reaction changes (switches) the aqueous base solubil-
ity of the irradiated regions. Major challenges in photolithog-
raphy, of particular relevance to the fabrication of ICs, are
simultaneously achieving high resolution (print small fea-
tures), with good pattern fidelity (printed feature matches
the design) and high sensitivity. Recent research efforts have
focused on achieving these aims for printed features with
dimensions of 32 nm or less; however, acceptable pattern fi-
delity has been elusive using conventional photoresist formu-
lations.10 The pattern fidelity can be quantified by measuring
the average deviation, to three standard deviations, of a
printed line edge compared to an ideal line edge (i.e., line
edge roughness [LER]).11 LER has been predicted to signifi-
cantly degrade device performance at small feature sizes.12
A
major contributing factor to LER is diffusion of photoacids
produced during irradiation of the photoresist.13,15 Random
diffusion of the photoacid results in a nonuniform distribu-
tion of deprotection reactions that ultimately results in fea-
tures with rough edges.16,18
One approach to overcoming problems associated with pho-
toacid diffusion is to use photoresists that do not use a PAG,
Additional Supporting Information may be found in the online version of this article.
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but instead rely on direct photolysis of bonds in the back-
bone of the photoresist polymer to facilitate a switch in the
polymers solubility. This class of resists, chain-scissioning
resists, were widely studied for lithography at longer wave-
lengths,19,25 however, after the development of resists that
use PAGs,9,26 commercial demand for manufacturing using
this process diminished. More recently, however, as LER and
high resolution patterning have become more important, in-
terest in evaluating chain-scissioning resists has been
renewed for 193 nm lithography and extreme ultraviolet li-
thography (EUVL). For example, the development and evalu-
ation of chain scissioning positive tone resists based on
methacrylates,27,28 polycarbonates,29,31 poly(olefin sul-
fones),32,36 acetal-containing polymers,37 molecular glasses,38
and halogenated polymers39,41 have recently been reported
for 193 nm and EUVL. Chain-scissioning resists have been
shown to have excellent resolution in these applications. For
example, trenches with widths between 3 and 10 nm were
achieved using poly(methyl methacrylate) (PMMA) as a pho-
toresist for electron-beam lithography42,45 and image quality
was shown to be excellent using EUV interference lithogra-
phy.27 As a comparison, the resolution of photoresist formu-
lations that use a PAG is limited to approximately 20 nm and
image quality parameters, such as LER, are relatively poor.46
A drawback of chain-scission resists is poor sensitivity, but
increasing the absorbance of the polymers at the wavelength
used for patterning, can improve sensitivity.30,34 More impor-
tantly for chain-scissioning resists is the requirement for de-
velopment with organic solvents. This is undesirable because
solvent developers are less compatible with manufacturing
tools, can cause swelling of the polymer, and are less envi-
ronmentally friendly.
repeat units derived from allyl benzene were found to
increase the sensitivity to photo-induced chain scission at
193 nm, but inhibited depolymerization in a subsequent bak-
ing step.34 The polymers used in this study have been rede-
signed so that the absorbing repeat units do not interfere
with depolymerization. The patterning performance of poly(-
olefin sulfone)-based resists was assessed using 193 nm in-
terference lithography and electron-beam lithography.
EXPERIMENTAL
Materials
Anhydrous sulfur dioxide (99%, BOC), 5-norbornen-2-ol
(99%, Aldrich), allylbenzene (98%, Aldrich), 3-(bicy-
clo[2.2.2]hept-5-en-yl)-1,1,1-trifluoro-2-(trifluoromethyl)pro-
pan-2-ol (98%, SynQuest) (NBHFA), bicyclo[2.2.1]hept-5-
ene-2-carboxylic acid (98%, Aldrich), ethyl chloroformate
(97%, Aldrich), 2-nitrobenzyl alcohol (Fluka), pyridine
(99%, Aldrich), 4-dimethylaminopyridine (99%, Aldrich), 3-
chloropropiophenone (98%, Aldrich), triethylamine (99%,
Aldrich),
1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (Alfa Aesar), sodium bicarbonate (99.5%,
Aldrich), sodium chloride (99.5%, Scharlau), hydrochloride
acid (UNIVAR, 32%), (GC) n-hexane (Merck. 98%), (GC)
diethyl ether (Merck, 99.7%), (GC) dichloromethane (Merck,
99.7%), (GC) chloroform (Merck, 99.4%), potassium t-butox-
ide (97%, Aldrich), (GC) tetrahydrofuran (THF) (Merck,
99.9%), cyclohexanone (Aldrich, 99.9%), (HPLC) 4-Methyl-
2-pentanone (Aldrich, 99.5%), (AR), ethyl acetate (UNIVAR),
(AR), methanol (UNIVAR), (AR), 2-propanol (UNIVAR), (AR),
Methyl isobutyl ketone (UNIVAR), (AR), hydrochloric acid
(UNIVAR, 32%), and (HPLC) tetrahydrofuran (LAB-SCAN,
99.8%) were obtained at the highest purity available and
used without further purification unless otherwise stated.
Tetramethylammonium hydroxide (TMAH, NMD-3, 2.38%)
was purchased from Tokyo Uhka Kogyo, Co. and used with-
out further purification unless stated. BARC29A bottom
antireflective coating solutions were obtained from Brewer
Science.
Another option for achieving a solubility switch is to use a
photochemical reaction that is not reliant on PAGs to change
the polarity of the polymer. For example, photosensitive o-
nitrobenzyl esters have been used as pendant side chains to
function as a hydrophobic protecting group for hydrophilic
carboxylic acids.47,49 Photoresists using this chemistry were
evaluated for lithography using 248 nm light.47,49 While the
quality of the patterning was good, the sensitivity of the
resists was poor compared to resists using PAG chemistry.
This chemistry has not been evaluated for 193 nm or EUVL.
Bicyclo[2.2.1]hept-5-en-2-yl Ethyl Carbonate
Ethyl chloroformate (17.5 mL, 18.3 ꢀ 10ꢁ2 mol) was added
in drops to a stirred solution of the 5-norbornene-ol (6.8 ꢀ
10ꢁ2 mol), pyridine (22 mL, 27.2 ꢀ 10ꢁ2 mol), and 4-dime-
thylaminopyridine (40 mg) in tetrahydrofuran (300 mL) at 0
ꢂC. The resulting cloudy mixture was stirred for 12 h at
room temperature (25 ꢂC). Brine (60 mL) and diethyl ether
(20 mL) were added, and the organic layers were separated,
washed successively with 10 vol/vol% HCl(aq) and brine,
dried over magnesium sulfate, and concentrated to leave the
crude product. Column chromatography over silica gel (10%
ethyl acetate/90% n-hexane) provided the pure bicy-
clo[2.2.1]hept-5-en-2-yl ethyl carbonate as a clear, colorless
oil. Yield 85%. 1H NMR (CDCl3, 500 MHz, ppm): d 0.87 (m),
1.26–1.31 (m), 1.45 (m)), 1.46 (m), 1.57 (m), 2–2.15 (m),
2.82 (s), 2.93(s), 3.15 (s), 4.1 (m), 4.5 (m), 5.18 (m), 5.94–
5.99 (m), 6.21–6.33 (m). 13C NMR (CDCl3, 500 MHz, ppm): d
14.7, 34.4, 40.4, 42.1, 45.7, 46.0, 47.5, 63.6, 78.4, 78.7, 131.2,
132.3, 138.7, 141.3, 141.3, 155.0.
Here a resist platform that does not have PAGs is introduced,
where the polymers have been designed to use both molecu-
lar weight and polarity solubility switches. The polarity
switch occurs through photolysis of pendant nitrobenzyl
esters, and chain scission is achieved through the use of a
poly(olefin sulfone) backbone, because this is inherently sen-
sitive to chain scission by radiation.50,52 Spectroscopic and
empirical evidence for the photochemistry at 193 nm is pre-
sented. Another attractive feature of poly(olefin sulfones) is
that following irradiation they can undergo depolymerization
when heated, that is, during a post-exposure baking (PEB)
step, which can increase the effective sensitivity of the
resist.32,53 The ability to undergo depolymerization depends
on the structure of the repeat units. For example, absorbing
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Synthesis of 2-Nitrobenzyl Bicyclo[2.2.1]hept-5-ene-2-
carboxylate
In a round-bottle flask, bicyclo[2.2.1]hept-5-ene-2-carboxylic
acid (1 eq.), (2-nitrobenzyl)methanol (1 eq.), triethylamine
covering the molar mass range of 1060–1,320,000 Da. FTIR
spectra of the thin films on silicon wafers were obtained
using a Nicolet Nexus 5700 FTIR spectrometer (Thermo
Electron Corp., Waltham, MA) equipped with a Harrick graz-
ing angle attenuated total reflectance (GATR) accessory (Har-
rick Scientific Products, Pleasantville, NY) fitted with a KRS-5
MIR polarizer (Harrick Scientific Products, Pleasantville, NY).
p-Polarized illumination was used. Spectra were recorded at
4 cmꢁ1 resolution for at least 128 scans with an optical path
difference (OPD) velocity of 1.899 cm sꢁ1. The thin film side
of the Si wafer was pressed directly onto the germanium in-
ternal reflection element of the ATR accessory, and a pres-
sure of 56 lbs inꢁ2 was applied. Spectra were manipulated
using the OMNIC 7 software package (Thermo Electron
Corp., Waltham, MA). Optical properties of the thin films, phi
and delta, were measured using a J.A. Woollam VUV-VASE. A
Cauchy model calculated film thickness, while an oscillator
model was used to model refractive index and absorbance.
(1.2
eq.),
1-(3-dimethylaminopropyl)-3-ethylcardiimide
hydrochloride (1.1 eq.), and 4-(dimethylamino)pyridine (0.1
eq.) were dissolved in dichloromethane. The reaction mixture
was stirred at room temperature overnight. Afterwards, the
mixture was washed three times with 1 N HCl(aq), saturated
NaHCO3(aq), and distilled water. The product was then
recrystallized from 70:30 hexane:ethyl acetate (vol:vol),
yielding yellowish solid (yield is ꢃ80%). The compound was
1
characterized by H NMR (CDCl3) 8.09 (CH), 7.71 (CH), 7.60
(CH), 7.43 (CH), 6.18 (CH), 5.88 (CH), 5.43 (CH2), 3.0 (CH),
2.52 (CH), 1.95 (CH), 1.57–1.28 (CH); 13C NMR (CDCl3) 174
(C¼¼O), 138 (CAN), 134 (C¼¼C), 133 (C¼¼C), 132 (C¼¼C), 130
(C¼¼C), 129 (C¼¼C), 128 (C¼¼C), 125 (C¼¼C), 63 (CAC), 50
(CAC), 46 (CAC), 43 (CAC), 42 (CAC), 29 (CAC).
Polymerization of Sulfur Dioxide with 2-Nitrobenzyl
Bicyclo[2.2.1]hept-5-ene-2-carboxylate
Resist Evaluation
Photoresist solutions were prepared by dissolving the poly-
mers (typically 2 wt %) in cyclohexanone. The solutions
were then spin-coated onto a BARC29A coated silicon wafer
at 1500 rpm approximately 70 nm thick before a 120 ꢂC
post-apply bake (PAB) step. The wafers were then exposed
to varying doses using a 193 nm ArF excimer laser (GAM
Laser, Orlando, FL). Afterwards, the wafers underwent a PEB
and were developed in 2.38 wt % TMAH solution. For spe-
cific details of the temperatures, see the Results and Discus-
sion section. The contrast curves were normalized to the ini-
tial film thickness. Patterning was done by dry 193 nm
interference lithography using an Amphibian XIS microstep-
per tool (Amphibian at Rochester Institute of Technology,
USA) using a 0.32 numerical aperture.54
In a typical polymerization, a mixture of 2-nitrobenzyl bicy-
clo[2.2.1]hept-5-ene-2-carboxylate (NPBHC) (1 g, 3.66 mmol)
in THF (2.67 mL) was transferred to a 50-mL Schlenk tube
and deoxygenated by three successive freeze-evacuate-thaw
cycles. The reaction vessel was then cooled with liquid nitro-
gen, and sulfur dioxide (ꢃ25 mL) was added by condensa-
tion. The polymerization was carried out at ꢁ15 ꢂC to ꢁ20
ꢂC for 2 h and was initiated with t-butyl hydroperoxide.
Afterwards, the reaction vessel was warmed to room temper-
ature and the polymer was purified through precipitation in
diethyl ether. All other polymerizations were similarly car-
ried out but with the addition of different monomers and
compositions.
Polymer Characterization
Electron-beam lithography (EBL) was carried out using a
Raith-150 system manufactured by Raith GmbH. Specifically,
the wafers with a native oxide layer (2 nm) that were coated
with 50 nm of the specified resist (Polymer B1, Polymer C,
or Polymer D [PMMA]) were exposed with an electron beam
with an acceleration voltage of 10–20 keV, an aperture of 20
lm, a 6 mm working distance, and a beam current of ꢃ100–
200 pA. Lithographic patterns consisting of line features
with critical dimensions of 11–50 nm with 100–200 nm
pitch and 100 lm long were designed using L-Edit Pro V.14
(Tanner EDA tool) and scanned with an area dose of 380 lC
cmꢁ2 at 20 kV for Polymer B1, single pass line dose of 120
pC cmꢁ1 at 20 kV for Polymer C, and single pass line dose of
375 pC cmꢁ1 at 10 kV for Polymer D [PMMA]. The samples
were then baked at the prescribed temperature for 60 s on a
hot plate followed by development in 2.38 wt/vol% TMAH
for 30 s at room temperature, followed by rinsing with
deionized water for 15 s, and drying with a jet of N2 (except
for the PMMA, which did not have a PEB and for which de-
velopment was performed using MIBK:IPA 1:3 for 30 s). Pat-
tern fidelity was examined by scanning electron microscopy
(SEM) at a magnification of 50,000 or 100,000ꢀ at 2 kV,
with an aperture of 30 lm and a working distance of 3 mm.
The LER of the patterned features was analyzed using
1H NMR spectroscopy was carried out using a Bruker Avance
DRX 500 spectrometer operating at 500.13 MHz for protons
and equipped with a 5 mm triple resonance z-gradient
probe. Deuterated chloroform (CDCl3) was used to dissolve
the organic samples. An internal standard, either tetrame-
thylsilane (TMS) or the residual proton signal of the deuter-
ated solvent, was used. Thermogravimetric aꢁn1alysis (TGA)
was performed at a heating rate of 10 ꢂC min in N2 on a
METTLER TOLEDO STARe thermogravimetric analyzer. Differ-
ential scanning calorimetry (DSC) was performed at a heat-
ing rate of 10 ꢂC minꢁ1 on a METTLER TOLEDO STARe dif-
ferential scanning calorimeter. Molecular weights of
polymers were measured using gel permeation chromatogra-
phy. The chromatographic system consisted of a 1515 iso-
cratic pump (Waters), a 717 autosampler (Waters), Styragel
HT 6E, and Styragel HT 3 columns (Waters) run in series, a
light scattering detector DAWN 8þ (Wyatt Technology
Corp.), and a 2414 differential refractive index detector
(Waters). THF was used as the mobile phase at a flow rate
of 1 mL minꢁ1. ASTRA (Wyatt Technology Corp.) and
Empower 2 (Waters) were used for data collection and proc-
essing. To determine molar mass by conventional SEC, the
columns were calibrated by polystyrene standards (Waters)
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Summit v7.5.1, a commercial lithography metrology software
package from EUV Technology (Martinez, CA). The LER was
determined along the full length of the line using a polyno-
mial edge detection algorithm with a threshold value of 0.5
determined by the average line threshold reference.
RESULTS AND DISCUSSION
Design of Poly(olefin sulfones) Resist Polymers
The aim of this study was to develop photoresist polymers
that do not require a PAG and function through two mecha-
nisms: a molecular weight reduction and also a polarity solu-
bility switch. In particular, after irradiation through a photo-
mask, the polymers should be able to be selectively
developed using a conventional aqueous base developer.
Scheme 1 shows a polymer structure undergoing photo-
induced chain scission and photodeprotection of a hydropho-
bic pendant group to yield a hydrophilic carboxylic acid. The
structures of the poly(olefin sulfones) investigated are
shown in Figure 1(A,B).
FIGURE 1 Structures of polymers used in this study.
Poly(olefin sulfones) can be prepared by free radical-initiated
polymerization of sulfur dioxide with alkenes. The 2-nitroben-
zyl bicyclo[2.2.1]hept-5-ene-2-carboxylate (NBHC) repeat unit
was selected because it contains an o-nitrobenzyl group,
which can undergo photoreaction to yield
a carboxylic
acid.48,55 The norbornene component of this repeat unit was
selected because the corresponding monomer undergoes
alternating polymerization with sulfur dioxide56 and the bicy-
clic structure should impart a higher Tg and etch resistance to
the final polymer.57 An alternating copolymer of sulfur dioxide
and NBHC was prepared by free radical polymerization, (Poly-
mer A), which had an absorbance of 7.7 lmꢁ1 at 193 nm.
This high absorbance is expected to degrade performance of
193 nm lithography by preventing sufficient light from pene-
trating the entire film thickness, so additional materials
including two additional norbornene-based monomers were
prepared for comparison. These monomers, bicyclo[2.2.1]-
hept-5-en-2-yl ethyl carbonate (BHEC) and 3-(bicyclo[2.2.2]-
hept-5-en-yl)-1,1,1-trifluoro-2-(trifluoromethyl)propan-2-ol
(NBHFA) are not expected to contribute significantly to the
absorbance of the polymer at 193 nm.58,59 Thus, by varying
the proportion of these non-absorbing monomers, it was
found that the absorbance of the polymer could be tuned
between 1.66 and 3.58 lmꢁ1 and varied linearly with the
NBHC content (see Table 1). This is significant because it has
been demonstrated that the absorbance of poly(olefin sul-
fone)s at 193 nm dictates the sensitivity to chain scission
when irradiated with this wavelength of light.34
It was also found that the refractive index of the polymers
systemically increases with increasing content of NBHC, lying
between 1.71 and 1.83 at 193 nm. Typically, the refractive
index of 193 nm photoresist polymers is 1.65–1.7. For 193
nm immersion lithography, an increased refractive index of
photoresists can precipitate gains in lithography parameters
SCHEME 1 Schematic diagram of a poly(olefin sulfone)-based
photoresist that functions through both a molecular weight
and polarity solubility switch.
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TABLE 1 Composition and Optical Properties of the Poly(olefin
sulfones) Used in This Study
b
Mn
Td
Abs
Polymer (kDa)a DM Composition (ꢂC) n193 nm (lmꢁ1
)
-
A
520
48
2.9 0:0:100
2.3 50:21:29
3.6 54:20:26
4.2 56:25:19
237 1.83
231 1.74
212 1.72
226 1.71
7.71
3.58
2.88
1.66
B1
B2
B3
a
43
47
Molecular weight parameters are reported relative to polystyrene
standards.
b
Td is measured at the onset of decomposition.
such as exposure latitude.60,61 The higher refractive index
observed here can be attributed to the nitrogen and sulfur con-
tent, as well as the nitrobenzyl ester group having an absorp-
tion edge close to 193 nm.62,66 Finally, BHEC is hydrophobic
and NBHFA is hydrophilic,58,59,67 hence, varying the proportion
of these monomers allows the polarity of the polymer and ulti-
mately the interaction of the polymer with the developer to be
controlled. The thermal decomposition temperature of all the
synthesized polymers (unirradiated) was above 200 ꢂC, thus
the polymers should be able to withstand typical lithographic
processing steps without thermal degradation.
FIGURE 2 GATR FTIR spectra of thin films of Polymer A after
exposure to 193 nm light at doses of 0–300 mJ cmꢁ2
.
try of the nitrobenzyl ester moiety at 193 nm has not previ-
ously been reported; the observed spectral changes
described above, that is, the loss of the nitro and ester
groups along with the formation of the carboxylic acids, are
nevertheless consistent with the mechanism reported for
longer wavelengths.
Polarity Switch—Photochemistry of Nitrobenzyl
Protecting Groups
The photodeprotection of nitrobenzyl groups by ultraviolet A
(UVA) and 254 nm light has been well studied,55,68 but the
behavior at 193 nm has not been investigated, for example,
whether the same photoreactions occur, or if there is a
change in the nature of photoproducts. Polymer A (see Fig.
1) was selected for in-depth studies to simplify spectroscopic
interpretation. GATR FTIR was used to follow the photo-
chemical changes that occur as a result of 193 nm irradia-
tion. GATR FTIR allows analysis of thin films or monolayers
coated on high refractive index substrates such as sili-
con,69,70 because of the enhanced electric field that occurs in
the nano-gap between the germanium internal reflective ele-
ment and the silicon wafer, when the film is less than ꢃ70
nm thick.
Figure 2 shows the GATR spectra of Polymer A after 193 nm
irradiation at doses ranging from 0 to 300 mJ cmꢁ2. Several
changes in the spectra can be observed with increasing irra-
diation dose. The peaks at 1526 and 1345 cmꢁ1 are charac-
teristic of the asymmetric and symmetric NO2 stretching
modes, respectively. These bands decreased with increasing
irradiation dose, with approximately 75% of the nitro groups
being removed at a dose of 300 mJ cmꢁ2. The carbonyl
stretching region also underwent changes. The ester C¼¼O
stretching band at 1734 cmꢁ1 can be observed to decrease
slightly and shift to 1722 cmꢁ1, consistent with the forma-
tion of carboxylic acids. Upon irradiation with UVA (ꢃ315–
400 nm)55,68 and 254 nm light, nitrobenzyl esters are
reported to undergo photoinduced reactions, in which the
benzylic CAO bond is cleaved to yield a carboxylic acid and
an o-nitrosobenzaldehyde (see Scheme 2). The photochemis-
SCHEME 2 Schematic shows
a reported mechanism for (i)
photo-induced formation of a biradical in the nitrobenzyl group,
(ii) subsequent chain transfer, and (iii) eventual deprotection to
form a pendent carboxylic acid.55,68 (iv) It is proposed that chain
scission of the poly(olefin sulfone) occurs as a consequence of
the formation of the radical species; that is, eventual abstraction
of a proton from a carbon on the polymer backbone results in
chain scission. (iv) Heating following chain scission then results
in depolymerization of the polysulfone backbone.
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respectively, reveals a decrease in intensity of these peaks
with increasing dose (a clearer 1D plot in color is shown in
Fig. S2 in the Supporting Information). After irradiation to
300 mJ cmꢁ2, approximately 40% of the sulfone groups are
lost. Purely aliphatic poly(olefin sulfones) such as poly(nor-
bornene sulfone), (PNBS) also undergo loss of sulfone
groups during 193 nm irradiation, but at the same dose only
10% of sulfone groups are lost.32 The increased loss of sul-
fone groups for Polymer A is attributed to its higher absorb-
ance, which is 7.7 lmꢁ1, compared with PNBS which has an
absorbance of only 0.17 lmꢁ1 32
Energy transfer from an
.
excited state nitrobenzyl group to the sulfone group is ruled
out as a possible mechanism, because there are no overlap-
ping allowed transitions. Previously, it has been shown that
incorporation of allyl benzene repeat units increases the 193
nm absorbance and also the sensitivity of the polymer to
lose sulfone groups at this wavelength. For allyl benzene
repeat units, the absorbing moiety is situated close to the
polymer chain and upon irradiation a radical is expected to
form on the carbon alpha to the aromatic ring.73 This radical
rearrangement is then likely to result in chain scission and
loss of sulfur dioxide. A recognized photoreaction pathway
for the nitrobenzyl group involves the formation of a reactive
biradical that can participate in hydrogen abstraction.55,68
For Polymer A, however, the nitrobenzyl absorbing unit is
separated from the backbone by a norbornene unit. To
explain the loss of sulfone groups mediated by the presence
of the nitrobenzyl groups, the radicals formed during irradia-
tion must undergo a number of radical chain transfer steps
and ultimately lead to abstraction of hydrogen atoms from
CAH on or adjacent to the polymer backbone and then con-
sequently to chain scission (see Scheme 2).
FIGURE 3 Absorbance spectra of thin films of Polymer A after
irradiation with various doses at 193 nm (0–300 mJ cmꢁ2). Op-
tical constants were measured using a variable angle spectro-
scopic ellipsometer and data was fit using an oscillator model.
Deprotection of the nitrobenzyl esters was also confirmed by
studying changes in the UV spectroscopic ellipsometry spec-
tra of the thin films as a function of 193 nm irradiation dose
(see Fig. 3). Before irradiation, a peak at 260 nm, due to the
p–p* transition of the nitrobenzyl functional group can be
observed. Upon irradiation, the intensity of this peak
decreases and bands at 300 and 350 nm appear and
increase in intensity with increasing dose. These later bands
are consistent with the formation of nitrosobenzalde-
hyde.71,72 This again is consistent with the mechanism
described above.
The deprotection reaction can also be followed by monitor-
ing the solubility of the polymer thin films in aqueous base
solution as a function of dose. Figure 4 shows a contrast
curve for Polymer A, which plots the relative film thickness
as a function of 193 nm dose, following a post-exposure
bake step at 120 ꢂC and development with a 2.38 wt %
aqueous solution of TMAH. Up to doses of approximately 30
mJ cmꢁ2, the film remains insoluble in the developer. At
higher doses, the rate of film loss increases until total disso-
lution was observed at approximately 110 mJ cmꢁ2. From
the GATR results, this corresponds to approximately 50%
deprotection of the nitrobenzyl groups at a dose of 110 mJ
cmꢁ2. The switch of the polymer solubility in the aqueous
base developer solution can primarily be attributed to the
formation of carboxylic acids, however chain scission during
the post exposure bake step also plays a role in dictating
polymer solubility (vide infra).
Molecular Weight Switch—Evidence for Chain Scission
Further inspection of the GATR spectra (Fig. 2) in regions
characteristic of sulfone groups, that is, the asymmetric and
FIGURE 4 Plot of relative film thickness versus 193 nm dose
for Polymer A, following a PEB step at 120 ꢂC and development
with 2.38% TMAH in water (contrast curves). Solid line shown
to guide the eye.
symmetric SO2 stretching modes at 1301 and 1134 cmꢁ1
,
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To enhance the sensitivity of this polymer, 15 wt % of acri-
dine, a known sensitizer,74 was added to the formulation.
Interestingly, addition of acridine only marginally increased
the absorbance at 193 nm by 0.1 lmꢁ1 (to 3.68 lmꢁ1),
although a peak with a maxima at 250 nm was observed to
appear (see Supporting Information Fig. S3). However, the
sensitivity of the formulation at 193 nm was found to signifi-
cantly improve. For example, Figure 5(b) shows that 1:1 150
nm lines could be resolved at a dose of 110 mꢁJ2cmꢁ2 (120
ꢂC PEB), which is an improvement of 60 mJ cm compared
to the performance without acridine. One cause of the
increase in sensitivity is likely to be due to energy transfer
from excited acridine molecules to the nitrobenzyl groups.
However, it is also worth noting that patterning was not
observed in the dose range used when the PEB step was
omitted. This indicates that increased depolymerization is
occurring in the irradiated regions during the PEB step and
may be due to the acridine acting as a chain transfer agent.
This result also shows that the molecular weight switch is
contributing to the sensitivity of the resists.
To investigate the PEB effect further, the effect of 193 nm
dose on line width was investigated for PEB temperatures of
ꢂ
120 and 130 C (Fig. 6). At both temperatures the line width
decreases with increasing dose. The reason for this is that
the deposition of photons in the resist is non-uniform. Spe-
cifically, the intensity of light varies sinusoidally across the
wafer, because by nature interference lithography occurs at
the diffraction limit. A similar intensity variation can also be
observed for lens-based manufacturing tools, because they
also operate at the diffraction limit when patterning at high
FIGURE 5 Top down SEM micrographs of 1:1 line spaces of (a)
Polymer B1 and (b) Polymer B1 with 15 wt % acridine that were
obtained by patterning with 193 nm interference lithography
(0.32 NA) and development with aqueous TMAH (2.38 wt %).
As well as loss of sulfone groups, the solubility of PNBS
increases in organic solvents with increasing dose and this
was interpreted as being due to a decreases in polymer mo-
lecular weight that was a consequence of photo-induced
chain scission reactions.32 Similar solubility studies for Poly-
mer A were confounded by the concurrent deprotection of
the nitrobenzyl units, which also changes polymer solubility.
Unfortunately, it was not possible to confirm the changes in
molecular weight using size exclusion chromatography (SEC),
because the thin films (70 nm) used in the irradiation study
did not provide sufficient polymer for a measurement. The
increased polarity of the polymer would also make compara-
tive SEC analysis challenging. Nonetheless, GATR-FTIR results
presented above provided evidence for chain scission and
this is further strengthened by the occurrence of depolymer-
ization during the post-exposure bake step (vide infra).
Patterning Performance and Evidence for
Depolymerization
Initially, patterning performance of Polymer B1 (see Table 1)
was assessed using 193 nm interference lithography. The
process involved exposing different regions on a wafer to a
FIGURE 6 Plots of line width (by 193 nm interference lithogra-
phy) versus dose for PEB temperatures of 120 (filled circle) and
130 ꢂC (crossed open square). The solid line is the linear
regression for the 120 ꢂC PEB data series (r2 ¼ 0.89, slope ¼
ꢁ0.8 6 0.2 nm cm2 mJꢁ1); the dotted line is the linear regres-
sion for the 130 ꢂC PEB data series (r2 ¼ 0.97, slope ¼ ꢁ1.3 6
0.1 nm cm2 mJꢁ1).
ꢂ
range of doses, employing a PEB at 120 C and development
with a standard aqueous base solution (2.38% TMAH(aq)).
Under these conditions, 150 nm lines and spaces could be
resolved at 170 mJ cmꢁ2 [see Fig. 5(a)]. Qualitatively, the
LER was high, but could not be quantified due to poor image
contrast. Clearly, the sensitivity of the system was also poor.
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FIGURE 7 Plots of LER versus normalized dose obtained following 193 nm interference lithography (0.32 NA) after development
with aqueous TMAH (2.38 wt %) (a) for Polymer B1 with added acridine (15 wt %) using a 120 ꢂC PEB step (filled circle), 130 ꢂC
PEB step (crossed open square), and (c) a commercial 193 nm photoresist.
resolution. This variation in light intensity generates chemi-
cal gradients in the latent image formed in the resist film.75
When the wafer is treated with developer, the soluble
regions of polymer are selectively removed to reveal a physi-
cal pattern. There are regions of the remaining polymer film
that have been exposed to light, but the local dose is insuffi-
cient to cause a solubility switch. However, when the total
dose is increased, the chemical gradient formed changes,
decreasing the observed line width after development. A
similar decrease in line width was also observed as a func-
tion of increasing dose for a PAG-based commercial photore-
sist (see Supporting Information Fig. S4). This decrease of
line width with increasing dose can be useful for double pat-
terning,76 a multistep process that is used to achieve high re-
solution patterning at resolutions well below the diffraction
limit.
tion process. Consequently, gains in sensitivity are possible
during a PEB step when appropriate absorbing units are
selected.
To determine how the depolymerization step influences the
quality of patterning, the LER values were determined as a
function of dose and PEB temperature. Figure 7(a) shows
plots of LER as a function of normalized dose for PEB tem-
peratures of 120 and 130 ꢂC. For the 130 ꢂC PEB tempera-
ture, LER clearly decreases with increasing dose, while for
the 120 ꢂC PEB temperature this was less pronounced,
although LER is significantly lower at the highest dose. This
behavior differs from that observed for a conventional 193
nm PAG-based photoresist, for which the LER remains essen-
tially unchanged with increasing dose [Fig. 7(b)]. It is
hypothesized that LER decreases with increasing dose for
Polymer B1 because two orthogonal chemical gradients are
established upon irradiation. The first is a gradient of car-
boxylic acids that are formed due to photodeprotection of
the nitrobenzyl esters, and the second is a gradient of chain-
scissioned polymer chains. The carboxylic acids should be
unaffected by the PEB, however, the polymer chains that
have undergone chain scission during irradiation will have
radicals present that will initiate depolymerization on heat-
ing. Higher temperatures result in increased depolymeriza-
tion.50 The observation of decreased LER and increased rates
of line-width shrinkage at higher PEB temperatures demon-
strates that depolymerization can play a role in tuning the
lithographic performance of these materials. The difference
between the dose dependence of LER for the poly(olefin sul-
fone) and chemically amplified resists (CARs) may be due to
the inherent differences in the physical and chemical proc-
esses involved. CARs rely on the catalytic deprotection of
hydrophobic esters by a strong acid to yield hydrophilic car-
boxylic acids. Diffusion of the acid catalyst is a significant
contributor to LER values.13,15 On the other hand, depoly-
merization of poly(olefin sulfones) is a radical process, in
Also of note in Figure 6 is the differencꢂe in the gradients of
the lines of best fit for the 120 and 130 C data series, which
are ꢁ0.8 6 0.2 and ꢁ1.3 6 0.1 nm cm2 mJꢁ1, respectively.
The gradient quantifies the amount of line-width shrinkage
per unit dose for the different PEB temperatures. The differ-
ence in the gradients can be attributed to the different rates
of depolymerization of the poly(olefin sulfone) backbone at
the two different PEB temperatures. Further evidence for de-
polymerization was obtained by comparing the GATR spectra
of thin films of Polymer B that had been irradiated to a dose
of 100 mJ cmꢁ2 and then annealed to 120 ꢂC, where a
decrease of sulfone and olefin repeat unit peaks was
observed (see Supporting Information Fig. S3). A decrease in
both peaks occurs as a result of the volatilization of both
species at the elevated temperatures experience during the
PEB step. Previously, the inclusion of certain absorbing co-
monomers, such as allyl benzene, has been shown to inhibit
depolymerization.34 Observing
a depolymerization effect
here indicates that absorbing groups such as nitrobenzyl
esters can be included without inhibiting the depolymeriza-
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FIGURE 8 Top-down SEM micrographs of approximately 32 nm trenches patterned in (a) Polymer B1, (b) Polymer D (PMMA), (c)
Polymer C with 8.41 wt % triphenyl sulfonium triflate, and 1.44 wt % trioctyl amine using EBL.
which the small molecule products (sulfur dioxide and
alkene) are non-radical, making their diffusion unimportant.
Furthermore, depolymerization is initiated by radicals on the
polymer chain, and can involve chain transfer of radicals
between polymer chains. These processes are controlled by
the dynamics of the polymer chain, as well as the ceiling
temperature of the polymer. The length scales involved
should be much less than those in PAG diffusion. Hence, the
depolymerization reactions will not significantly spread
beyond the regions of polymer that have been exposed to
light and this is a possible reason why reduced LER was
observed under certain conditions.
base development, which is better suited to manufacturing
tools and more environmentally friendly.
CONCLUSIONS
A novel photoresist platform based on a poly(olefin sul-
fone) polymer backbone functionalized with pendant nitro-
benzyl esters was synthesized and its performance exam-
ined. The results from GATR FTIR and UV spectroscopic
ellipsometry of the polymer thin films before and after 193
nm irradiation were consistent with the polymer under-
going both chain scission and also photodeprotection of the
hydrophobic nitrobenzyl ester to yield hydrophilic carbox-
ylic acids that were soluble in aqueous base. Following irra-
diation, depolymerization was also observed during an
annealing step that served to increase the sensitivity of the
resists. Patterning was possible using 193 nm interference
lithography during which sensitivity increased as a result of
using a photosensitizer (acridine). Depolymerization of the
polymer was found to improve the LER during patterning
when higher doses were used. High resolution patterning
was carried out using EBL. While sensitivity could still be
increased, LER was superior to an open source EUVL resist
that used PAG chemistry. The patterning performance was
comparable to PMMA, but with the advantage that aqueous
developers could be used.
High Resolution Patterning Performance
EBL was used to test the high resolution patterning perform-
ance of Polymer B1. EBL can also be used as a good model
for EUV lithography, because when EUV photons (13.5 nm,
92 eV) interact with a polymer, secondary electrons are re-
sponsible for the photo-induced chemical changes in the
photoresist.77 In addition, both EBL and EUVL are conducted
under high vacuum conditions. EBL is extensively used in
the manufacture of photomasks,2 and multiple electron beam
approaches78 have also been proposed as alternative next
generation lithography platforms. Figure 8 shows the top-
down SEM micrographs for EBL patterning of trenches with
a resolution of approximately 32 nm for Polymer B1 (30.5
nm trenches), Polymer D (PMMA, a commonly used e-beam
resist; 33.6 nm trenches), and Polymer C, an open source
EUVL resist polymer (32.6 nm trenches) (see Fig. 1 and Ta-
ble 1 for compositions). The poly(olefin sulfone) and PMMA
had comparable LER values of 2.7 6 0.6 nm and 3.0 6 0.2
nm, respectively, while the open source EUVL resist formula-
tion (Polymer C) had a much higher LER of 7.4 6 0.9 nm.
The LER value for this EUVL resist formulation is similar to
values reported in the literature for patterning using EUVL.79
Note that the minimum LER typically observed for optimized
PAG-based EUVL resists, is approximately 4 nm.80 The
results here indicate that chain-scissioning resists can
achieve much better LER values than standard resists for
high resolution patterning. This is because the resists
reported here do not rely on diffusion of a photoacid to
cause the switch in polymer solubility. The performance of
Polymer B1 was comparable to that of PMMA, however, an
advantage of Polymer B1 was compatibility with aqueous
ACKNOWLEDGMENTS
This research was supported under the Australian Research
Council’s (ARCs) Linkage Projects Scheme (project number
LP0882551), with SEMATECH as a financial industry partner. I.
Blakey and A.K. Whittaker would also like to acknowledge the
ARC for a Future Fellowship (FT100100721) and Australian
Professorial Fellowship, respectively. Equipment used in this
research was supported by the ARCs Linkage Equipment, Infra-
structure and Facilities funding schemes (project numbers
LE0668517 and LE0775684). This work was performed in part
at the Queensland and Canberra nodes of the Australian
National Fabrication Facility. This work was performed in part
at the Bio-Nano Development Facility, which was funded by the
Queensland State Government Smart State Innovation Building
Fund. Authors acknowledge Lauren Butler for performing 193
nm laser and VUV-VASE measurements.
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28 Baylav, B.; Zhao, M.; Yin, R.; Xie, P.; Scholz, C.; Smith, B.;
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